Operating Systems, 32 vs 64-bit, skas, performance
- Can I run UML on Microsoft Windows, FreeBSD or Solaris? etc
- No. UML relies on the Linux kernel (it would theoretically be possible to port it to other platforms but this is unlikely to ever happen)
- Can I run a 32-bit UML kernel on a 64-bit host? 32-bit host?
- Yes. When compiling on a 64-bit host, use the switch
make ARCH=um SUBARCH=i386
- Can I run a 64-bit UML kernel on a 64-bit host? 32-bit host?
- You can only run a 64-bit UML kernel on a 64-bit host.
Note: 64-bit UML kernels do not support 32-bit binaries.
- Can I use SMP? (more than one CPU)
- What is UML good at?
- UML can be quite useful for testing, kernel development and debugging, isolating a process, education, etc. (more uses)
It has been used for virtual hosting in the past, but modern virtualization solutions have supplanted it in this area, see above.
- How fast is it compared to other virtualization technologies
- If performance is your primary concern, you should probably use something else.
Open-source solutions include:
- QEMU: "QEMU is a generic and open source machine emulator and virtualizer" - actually slower than UML..
- KVM: accelerated version of QEMU for supported CPUs
- VirtualBox: originally based on QEMU, has non-free/non-open-source extensions
- Lguest: "allows you to run multiple copies of the same 32-bit kernel" via a kernel module.
- What is skas? skas3, skas0, tracing thread?
- UML was originally written with a single tracing thread, this was very slow so the skas3 patch was designed to provide a major speed boost. Unfortunately it was never merged into the mainline kernel and it is no longer supported. All recent kernels support skas0, which is almost as fast.
Usage: gdb, modules, SELinux, disk formats
- How can I debug with GDB?
- compile your kernel with
- start the process in gdb, or attach to an existing UML process
- tell gdb to ignore SIGSEGV:
handle SIGSEGV noprint nostop pass
- compile your kernel with
- Can I run SELinux inside UML
- Does UML understand VDI or VMDK disks?
- No. UML works with raw disks, stored as files or even real devices.
However, it will support all partition formats and filesystem formats that you compiled in.
VFS: Cannot open root device"
- Ensure you have specified a
ubda=root_fsoption to the kernel's command line
- I can't load any modules?
- You need to copy the modules you compiled into the filesystem image - the easiest way is to loop mount it.
- The system starts but I never get a login prompt
- A number of filesystem images have problems running under UML and may not start a shell login. Your options are:
- Try another image
- boot with
init=/bin/bash(no other process will be started and the root filesystem will still be read-only!)
- Loop mount the image prior to launching and ensure the network/sshd are configured so you can login via ssh